This type of coding/decoding is based on the extraction of spatialization parameters so that, on decoding, the listener's spatial perception can be reconstructed.
Such a coding technique is known by the name “Binaural Cue Coding” (BCC) which is on the one hand aimed at extracting and then coding the indices of auditory spatialization and on the other hand at coding a monophonic or stereophonic signal arising from a matrixing of the original multi-channel signal.
This parametric approach is a low-bitrate coding. The principal benefit of this coding approach is to allow a better compression rate than the conventional procedures for compressing multi-channel digital audio signals while ensuring the backward-compatibility of the compressed format obtained with the coding formats and broadcasting systems which already exist.
The MPEG Surround standard described in the document of the MPEG ISO/IEC standard 23003-1:2007 and in the document by “Breebaart, J. and Hotho, G. and Koppens, J. and Schuijers, E. and Oomen, W. and van de Par, S.,” entitled “Background, concept, and architecture for the recent MPEG surround standard on multichannel audio compression” in Journal of the Audio Engineering Society 55-5 (2007) 331-351, describes a parametric coding structure such as represented in FIG. 1.
Thus, FIG. 1 describes such a coding/decoding system in which the coder 100 constructs a sum signal (“downmix”) Ss by matrixing at 110 the channels of the original multi-channel signal S and provides, via a parameters extraction module 120, a reduced set of parameters P which characterize the spatial content of the original multi-channel signal.
At the decoder 150, the multi-channel signal is reconstructed (S′) by a synthesis module 160 which takes into account at one and the same time the sum signal and the parameters P transmitted.
The sum signal comprises a reduced number of channels. These channels may be coded by a conventional audio coder before transmission or storage. Typically, the sum signal comprises two channels and is compatible with conventional stereo broadcasting. Before transmission or storage, this sum signal can thus be coded by any conventional stereo coder. The signal thus coded is then compatible with the devices comprising the corresponding decoder which reconstruct the sum signal while ignoring the spatial data.
This coding scheme relies on a tree structure which allows the processing of only a limited number of channels simultaneously. Thus, this technique is satisfactory for the coding and the decoding of signals of reduced complexity used in the audiovisual sector such as for example for 5.1 signals. However, it does not make it possible to obtain satisfactory quality for more complex multi-channel signals such as for example for the signals arising from direct multi-channel sound pick-ups or else ambiophonic signals.
Indeed, such a structure limits the exploitation of the inter-channel redundancy which may exist for complex signals. Moreover, multi-channel signals exhibiting phase oppositions, such as for example ambiophonic signals, are not well reconstructed by these techniques of the prior art.
There therefore exists a requirement for a parametric coding/decoding technique for multi-channel audio signals of high complexity which makes it possible to manage at one and the same time the signals exhibiting phase oppositions and to take into account inter-channel redundancies between the signals while being compatible with a low bitrate coding.